In electrophotographic copying machines, the moving surface of a photoreceptor is charged to a substantially uniform potential during passage through a charging station, and then, during passage through an imaging station, the charged photoreceptor surface is exposed to a light image of an original document to be copied. The light image selectively discharges the photoreceptor surface to produce a latent electrostatic image replicating the original document. This latent image is then translated through a development station where it is developed by contacting the photoreceptor surface with developer powder or toner to create a powder image conforming to the latent electrostatic image. The powder image moves with the photoreceptor to a transfer station where it is transferred or offset printed onto a copy sheet. The transferred powder image is fused to the copy sheet surface, thereby creating a permanent copy of the original document. The photoreceptor then returns to the charging station through a cleaning station where residual toner is removed from the photoreceptor surface.
In the case where the development station apparatus uses a two-component developer material to develop the latent electrostatic image into a powder image, fine toner particles or powder are made to adhere to the surfaces of coarse, ferromagnetic carrier granules or beads by triboelectric attraction. This two-component developer is brought into contact with the photoreceptor surface by suitable means, such as a magnetic brush forming roller, to transfer toner from the carrier beads to the latent electrostatic image, thereby creating the conforming powder image. Unfortunately, some of the carrier beads may also be attracted to the photoreceptor surface and are held there with the powder image upon exiting the development station. This phenomenon, known as "bead carryout" can be a persistent problem in electrophotographic copiers. In the transfer station, these spurious, adhering carrier beads prevent localized intimate contact between the copy sheet surface and the toner particles of the powder image, thereby causing copy deletions that degrade copy quality. Moreover, if these spurious carrier beads remain attracted to the photoreceptor surface as it goes through the cleaning station, they can abrade the photoreceptor surface as they are mechanically removed by a rotating cleaning brush. It is therefore highly desirable that all such spurious carrier beads leaving the development station on the photoreceptor surface be removed prior to arrival at the transfer station. It is also desirable that the removal of these carrier beads from the photoreceptor surface be achieved without disturbing the powder image.
Another important consideration is that a carrier bead pick-off device structured to achieve these desired objectives be as compact as possible, since available space within the confines of an electrophotographic copier is typically at a premium.